A ball bearing as shown in FIG. 6 is widely used in the art so as to support various rotatable portions such as bearing portions of the various rotatable machines and apparatus. This ball bearing comprises an inner ring 2 having an outer peripheral surface on which an inner ring raceway 1 is formed, an outer ring 4 having an inner peripheral surface on which an outer ring raceway 3 is formed, with the inner and outer rings 2, 4 being concentrically arranged, a plurality of balls 5 rotatably disposed between the inner and outer ring raceways 1, 3, and a cage 6 having a plurality of pockets 8 for rotatably accommodating the balls 5, respectively.
In the case illustrated, the inner and outer ring raceways 1, 3 are of a deep groove type.
The cage 6 of the ball bearing illustrated in FIG. 6, which is referred to as wave shaped or corrugated press cage, is made of a metal plate member by press-forming, and comprised of a pair of elements 9a, 9b each formed in a wave-like or corrugated annular shape and combined together. Specifically, the element 9a is formed with recessed portions 8a arranged in a circumferential direction, while the element 9b is formed with recessed portions 8b arranged in a circumferential direction, and the recessed portions 8a, 8b cooperate to form the pockets 8. Disposed circumferentially between the recessed portions 8a, 8b are connecting portions, and the rating connecting portions are placed in contact with each other and fixed with a rivet 10, so that the cage 6 is formed in an annular shape to have the pockets 8 arranged in a circumferential direction.
The recessed portions 8a, 8b have an inner surface the intermediate portion of which forms a spherically concave portion 11 which has a radius of curvature slightly larger than the radius of curvature of the balls 5. Accordingly, the pair of elements 9a, 9b are abutted with each other to combine the recessed portions 8a, 8b to form the pockets 8.
The cage 6 illustrated in FIG. 7, which is referred to as crown shaped cage, comprises an annular main portion 7 made of a synthetic resin etc., and formed with a plurality of pockets 8 arranged in a circumferential direction for rotatably holding balls 5 (see FIG. 6), respectively.
In the case of crown type cage 6, the main portion 7 is formed with pairs of resilient projections 12, each pair of resilient projections 12 spaced apart from each other and having a concave side surface portion, respectively, with a spherical concave surface portion 13 therebetween, so that the concave side surface portions of the pair of the resilient projections 12 are opposed to each other. Each of the pockets 8 is defined by the pair of the concave side surface portions and the spherical concave surface portion 13 therebetween. The spherical concave surface portion 13 has a radius of curvature slightly larger than the radius of curvature of the outer surface of the balls 5.
Incidentally, the spherical concave surface portion 13 is faced in an axial direction of the bearing. The term "axial" means an up and down direction in FIG. 7, and the spherical concave surface portion 13 is faced upwards in FIG. 7.
When assembling the ball bearing, forcing the balls 5 into the pockets 8, respectively, through the space between the tip end edges of the pair of the resilient projections 12 which are resiliently pushed away, so that the space between the tip and edges of the pair of the resilient projections 12 is enlarged.
Thus, the balls 5 are nested in the pockets 8, respectively, and supported by the cage 6 between the inner ring raceway 1 and the outer ring raceway 3 (see FIG. 6).
During use of the ball bearing provided with the cage 6, the inner ring 2 and the outer ring 4 are relatively rotated due to the rotating balls 5, which spin around the inner ring 2. The cage 6 is rotated around the inner ring 2 with the same speed to the spinning speed of the balls 5.
Lubricant such as grease is filled in or supplied continuously to a portion between the outer peripheral surface of the inner ring 2 and the inner peripheral surface of the outer ring 4 so that the relative rotation as mentioned above is made smoothly, avoiding vibration and noise and preventing failures such as seizure in the ball bearing.
The ball bearing may be provided with a sealing member such as seal plate and shield plate to close the openings at the both ends of the space between the outer peripheral surface of the inner ring 2 and the inner peripheral surface of the outer ring 4, so that the lubricant is prevented from leaking out of the space, and any foreign matter such as dust is prevented from penetrating into the space, although FIG. 6 shows a ball bearing which is not provided with such a seal member.
The ball bearing having such a cage 6 installed therein occasionally experiences vibration and noise referred to as "cage noise" due to the vibrations caused in the cage 6 even when a necessary amount of lubricant is filled in or supplied to it. Such vibrations in the cage 6 are caused based on the sliding friction between the balls 5 and the cage 6 due to the relatively large amount of movements of the cage 6 with reference to the balls 5.
In order to prevent the cage noise from occurring, conventionally the clearance between the inner surface of the pockets 8 and the rolling surface of the balls 5 is made small to minimize the movement amount of the cage 6 with reference to the balls 5, so that the occurrence of the cage noise is refrained.
However, only with minimizing the movement amount of the cage 6 with reference to the balls 5, the cage noise are still produced due to the shape of the inner peripheral surface of the pockets 8 of the cage 6. The reason is explained referring to FIG. 8 and 9 as follows.
The pockets 8 of the cage 6 have peripheral edge portions 14 along the opposite openings, which have a sharp corner portion 15 having a large curvature which is reluctant to the flow of the lubricant.
It will be noted that it is difficult for the lubricant to enter into the clearance 16 between the inner surface of the pockets 8 and the rolling surface of the balls 5 when the clearance 16 is made small so as to refrain the cage noise. Specifically, the lubricant, when entering the clearance from the surrounding space as the balls 5 rotate, is scraped by the corner portions 15 and thus prevented from entering the clearance 16. Consequently, an insufficient amount of lubricant is supplied to the interior of the clearance 16, the friction and vibration in the sliding contact portions between the cage 6 and the balls 5 are not sufficiently refrained, thus vibration and noise are produced.
On the other hand, conventionally, there are cages having pockets the inner surface of which is formed in a radial cylindrical shape, not spherical, with its central axis placed in the radial axis of the cage. In the case of the cage having the pockets with the radial cylindrical inner peripheral surface, the lubricant attached to the rolling surfaces of the balls is not scraped by the peripheral edge portions along the openings of the pockets, the friction vibration is prevented from occurring in the sliding contact portion between the cage and the balls. However, the lubricant may be excessively taken in the pockets. As the excessive amount of lubricant exists in the pockets, the resistance against the rolling balls in the pockets is larger, so that the rotating torque of the radial bearing with the cage installed therein is larger.
In the case of the miniature bearings used for the radial bearings to be installed in the rotatable support portions of the compact motors etc., the increase of the rotating torque may seriously affect the performance such as cell life, wow flutter etc. of the devices having the compact motor installed therein. Therefore, still improvements are required.